Transcript Chem 1201 - LSU Department of Chemistry

Watkins
Chapter 8
CHAPTER 8
MOLECULES
Lewis Symbols - e- Configurations
Ionic Bonding - Salts
Covalent Bonding - Main Group Molecules
The Myth of the Octet “Rule"
Polarity, Electronegativity, Bond Strength
Lewis Structures of molecules
Resonance
Oxidation Numbers vs. Charge
Chemistry 1421
1
Brown & Lemay
Watkins
Chapter 8
Lewis Symbols - Atoms
Main Group Atoms only: [Core]nsxnpy
Example: Valence Shell (n) = 2, Core = [He]
1A
2A
2s1p0 2s2p0
Li
Be
3A 4A
5A
6A 7A
8A
2s2p1 2s2p2 2s2p3 2s2p4 2s2p5 2s2p6
B
C
N
O
F
Ne
2p
2s
Ground State Valence Shell Orbital Energy Level Diagrams
Chemistry 1421
2
Brown & Lemay
Watkins
Chapter 8
Lewis Symbols - Atoms
Main Group Atoms only: [Core]nsxnpy
Example: Valence Shell (n) = 2, Core = [He]
1A
2A
2s1p0 2s2p0
Li
Be
Be
Chemistry 1421
3A 4A
5A
6A 7A
8A
2s2p1 2s2p2 2s2p3 2s2p4 2s2p5 2s2p6
B
N
C
Prepare atoms for
bonding in compounds
by promoting electrons
to empty orbitals in the
valence shell.
B
C
3
O
F
Ne
Brown & Lemay
Watkins
Chapter 8
Lewis Symbols - Atoms
Main Group Atoms only: [Core]nsxnpy
Example: Valence Shell (n) = 2, Core = [He]
1A
2A
2s1p0 2s2p0
Li
3A 4A
5A
6A 7A
8A
2s2p1 2s2p2 2s2p3 2s2p4 2s2p5 2s2p6
Be
B
C
Be
B
C
Unpaired and paired electron
dots are significant!!
Chemistry 1421
4
N
O
F
Ne
Excited States
There is no significance in
the position of the dots!!
Brown & Lemay
Watkins
Chapter 8
Lewis Symbols - Atoms
Main Group Atoms only: [Core]nsxnpy
Example: Valence Shell (n) = 2, Core = [He]
1A
2A
2s1p0 2s2p0
Li
3A 4A
5A
6A 7A
8A
2s2p1 2s2p2 2s2p3 2s2p4 2s2p5 2s2p6
Be
B
C
Be
B
C
N
O
F
Ne
Excited States
The valence of an atom is equal to the maximum
number of chemical bonds the atom can make.
Chemistry 1421
5
Brown & Lemay
Watkins
Chapter 8
Lewis Symbols - Atoms
Main Group Atoms only: [Core]nsxnpy
Example: Valence Shell (n) = 2, Core = [He]
1A
2A
2s1p0 2s2p0
Li
3A 4A
5A
6A 7A
8A
2s2p1 2s2p2 2s2p3 2s2p4 2s2p5 2s2p6
Be
B
C
Be
B
C
N
O
F
Ne
Excited States
The valence of an atom is equal to the maximum
number of unpaired electrons on the Lewis Symbol.
Chemistry 1421
6
Brown & Lemay
Watkins
Chapter 8
Lewis Symbols - Atoms
Main Group Atoms only: [Core]nsxnpy
Example: Valence Shell (n) = 2, Core = [He]
1A
2A
2s1p0 2s2p0
Li
3A 4A
5A
6A 7A
8A
2s2p1 2s2p2 2s2p3 2s2p4 2s2p5 2s2p6
Be
B
C
Be
B
C
N
O
F
Ne
Excited States
1 2
3 4 3 2 1 0
S E C O N D R O W VALE N C E
Chemistry 1421
7
Brown & Lemay
Watkins
Chapter 8
Lewis Symbols - Atoms
Main Group Atoms only: [Core]nsxnpy
1A
2A
ns1p0 ns2p0
X
X
3A 4A
5A
6A 7A
8A
ns2p1 ns2p2 ns2p3 ns2p4 ns2p5 ns2p6
X
X
X
X
X
X
The ground state Lewis symbols shown above are the same for all
group members in the main group. For example, all 6A elements
(O down to Po) have the same Lewis symbol X
However, the excited state Lewis symbols, created when electrons
are promoted to empty valence shell orbitals in preparation for
bonding, are not the same.
Chemistry 1421
8
Brown & Lemay
Watkins
Chapter 8
Lewis Symbols - Atoms
3rd row non-metals: [Core]ns2npy
Example: Valence Shell (n) = 3, Core = [Ne]
4A
3s2p2
5A
3s2p3
6A
3s2p4
7A
3s2p5
8A
3s2p6
Si
P
S
Cl
Ar
3d
3p
3s
Ground State Lewis Symbols
Valence = maximum number of chemical bonds which can be formed.
= number of unpaired electrons.
Promote electrons to empty orbitals in the valence shell.
Chemistry 1421
9
Brown & Lemay
Watkins
Chapter 8
Lewis Symbols - Atoms
3rd row non-metals: [Core]ns2npy
Example: Valence Shell (n) = 3, Core = [Ne]
4A
3s2p2
5A
3s2p3
6A
3s2p4
7A
3s2p5
8A
3s2p6
Si
P
S
Cl
Ar
3d
3p
3s
Excited State Lewis Symbols
4
5
6
7
8
E X PA N D E D VA L E N C E
Chemistry 1421
10
Brown & Lemay
Watkins
Chapter 8
Lewis Symbols – Monatomic Ions
Most Main Group Ions have either:
G
G
[Core]ns2np6 (filled sp valence subshells)
or
[Core]ns0np0 (empty sp valence subshells)
Noble Gas configuration:
s2p6 = 8 electrons - "Octet" - is very stable
In ionic and molecular compounds, electron spinpairs are much more stable than unpaired electrons
Chemistry 1421
11
Brown & Lemay
Watkins
Chapter 8
Salts - Ionic Compounds
Na(s) + 12 Cl2(g) → NaCl(s)
Na
Cl
→
REDOX Reaction
gain and loss of electrons
E+
Lattice
Energy
Chemistry 1421
[ Na ][ Cl ]
[Ne]
[Ar]
[ Na ][ Cl ] → Na + Cl
solid
coulombic
forces
gas
12
Brown & Lemay
Watkins
Chapter 8
IONIC vs. MOLECULAR
Ions in salts
held together by IONIC bonds
(electrostatic or "Coulombic" attraction)
spin-pairs formed by electron transfer
Atoms in molecules
held together by COVALENT bonds
spin-pairs formed by electron sharing
Chemistry 1421
14
Brown & Lemay
Watkins
Chapter 8
Covalent Bond: H2
Valence
1
H: 1s1
1 valence subshell: 1s
1 valence orbital: s
H
H
+
H H (H H)
H
Covalent bond formed by Orbital Overlap
Each atom has a filled valence shell [He]
One shared spin-pair (
Chemistry 1421
) = Single Covalent Bond
15
Brown & Lemay
Watkins
Chapter 8
Covalent Bond: General Rules
Electrons spin-pair by sharing
Unpaired electrons are very rare, because
Free Radicals are highly reactive
Electrons in Valence Shell Orbitals ONLY
All orbitals in a valence shell may be used
Chemistry 1421
16
Brown & Lemay
Watkins
Chapter 8
Covalent Bond: F2
Valence
1
F
F: [He]2s22p5
2 valence subshells: 2s, 2p
4 valence orbitals: s, px, py, pz
F
+
F
F F
All electrons are spin-paired
Each atom has a filled valence shell (an octet)
Each atom has a noble gas configuration
Chemistry 1421
17
Brown & Lemay
Watkins
Chapter 8
Covalent Bond: HF
[He] [Ne]
H
H F
Bond
Pair BP
Chemistry 1421
F
H F
Lone
Pairs LP
H F
Lone pairs are sometimes
not shown.
18
Brown & Lemay
Watkins
Chapter 8
Covalent Bond: O2
Valence
2
O
O: [He]2s22p4
2 valence subshells: 2s, 2p
4 valence orbitals: s, px, py, pz
O
+
O
O O O O
Two shared pairs = Double Covalent Bond
Each atom has a noble gas configuration and
filled s-p subshells - an octet
Chemistry 1421
19
Brown & Lemay
Watkins
Chapter 8
Covalent Bond: N2
Valence
3
N
N
+
N: [He]2s22p3
2 valence subshells: 2s, 2p
4 valence orbitals: s, px, py, pz
N
N N
N N
Three shared pairs = Triple Covalent Bond
Each atom has a noble gas configuration and
four filled orbitals - an octet
Chemistry 1421
20
Brown & Lemay
Watkins
Chapter 8
Covalent Bond: CH4
Valence
4
C
[He]2s12p3, 2 subshells, 4 orbitals
H
H
H C H
H
H
H
C
CH4 Methane
H
Each atom has a noble gas configuration
Chemistry 1421
21
Brown & Lemay
Watkins
Chapter 8
Covalent Bond: C2H6
Valence
4
C
[He]2s12p3, 2 subshells, 4 orbitals
H
H
H
C
C
H
H
H
H H
C2H6 Ethane
H C C H
H H
Chemistry 1421
22
Brown & Lemay
Watkins
Chapter 8
Covalent Bond: C2H4
Valence
4
C
[He]2s12p3, 2 subshells, 4 orbitals
H
H
Chemistry 1421
H
C
C
H
H
C2H4 Ethene (Ethylene)
C C
H
H
H
23
Brown & Lemay
Watkins
Chapter 8
Covalent Bond: C2H2
Valence
4
C
[He]2s12p3, 2 subshells, 4 orbitals
H C
H C C H
Chemistry 1421
C H
C2H2 Ethyne (Acetylene)
24
Brown & Lemay
Watkins
Chapter 8
Covalent Bond:
Valence
4
C
[He]2s12p3, 2 subshells, 4 orbitals
e-
C C
Chemistry 1421
2C2
C
C
e-
C22- Carbide Anion
(same Lewis diagram
as N2)
25
Brown & Lemay
Watkins
Chapter 8
Molecules and Molecule Ions
Main Group Non-Metals (and some semi-metals)
What orbitals are used in bonding?
n = 1 H: 1 orbital in V.S. (He forms no bonds!)
(s2) duet always
H
Since H forms only one bond, it is monovalent , and
has a valence of 1.
"Valence" means "combining number", which is the
same as the maximum number of shared-pair
covalent bonds the atom can make.
Chemistry 1421
26
Brown & Lemay
Watkins
Chapter 8
Molecules and Molecule Ions
Main Group Non-Metals (and some semi-metals)
What orbitals are used in bonding?
n = 2 C, N, O, F: 4 orbitals in V.S.
(s2p6) octet always
C
C
C
C
Carbon is tetravalent (valence = 4) always!
Chemistry 1421
27
Brown & Lemay
Watkins
Chapter 8
Molecules and Molecule Ions
Main Group Non-Metals (and some semi-metals)
What orbitals are used in bonding?
n = 2 C, N, O, F: 4 orbitals in V.S.
(s2p6) octet always
N
N
N
Nitrogen is trivalent
F
O
O
Oxygen is divalent
Chemistry 1421
Fluorine is monovalent
28
Brown & Lemay
Watkins
Chapter 8
Molecules and Molecule Ions
Main Group Non-Metals (and some semi-metals)
What orbitals are used in bonding?
n = 2 C, N, O, F: 4 orbitals in V.S.
(s2p6) octet always
C
N
N
C
C
C
N
O
O
F
C, N, O and F always obey an “octet rule”!
Chemistry 1421
29
Brown & Lemay
Watkins
Chapter 8
Molecules and Molecule Ions
Main Group Non-Metals (and some semi-metals)
What orbitals are used in bonding?
n = 2 B: in many molecules < octet
"electron deficient"
B
B
Boron is normally trivalent, so it does not normally
obey an octet rule.
But sometimes it is tetravalent (“coordinate covalent
bond”), so it sometimes obeys an octet rule.
Chemistry 1421
30
Brown & Lemay
Watkins
Chapter 8
Molecules and Molecule Ions
Main Group Non-Metals (and some semi-metals)
What orbitals are used in bonding?
n > 2 s2p6 octet (sometimes)
s2p6dx > octet (sometimes)
Expanded valence is not a “violation” of
an Octet Rule.
Many examples later
Chemistry 1421
31
Brown & Lemay
Watkins
Chapter 8
Myth of the Octet Rule
Main Group monatomic ions often achieve a noble
gas configuration (He duet or Ne-Xe octet).
In molecules, an “octet rule” is obeyed rigorously
by only four (4) elements: C, N, O, F
The reason the noble gas configuration seems to be
so important is that >90% of all known
compounds contain just C, H, N and O.
The “octet” (noble gas configuration) is a helpful
guide (but not a rule) for understanding other ions
and atoms in molecules.
Chemistry 1421
32
Brown & Lemay
Watkins
Chapter 8
Myth of the Octet Rule
The “octet rule” is obeyed rigorously by only four
(4) elements: C, N, O, F
The real rules for molecule formation are:
1. Electrons form spin-pairs.
2. The maximum possible valence for an n-th row
element is n2, the number of available orbitals.
3. All valence shell electrons and orbitals may be
used for bonding, ...
4. ... but for n > 2, the observed valence is usually
less than n2 but often more than 4.
Chemistry 1421
33
Brown & Lemay
Watkins
Chapter 8
Electronegativity
An atomic property with a periodic trend used to
understand the structure and properties of molecules.
Electron Affinity measures atom-electron
attraction in a gas (pg 276)
the more negative EA, the stronger the attraction
(both positive and negative values)
Electronegativity measures atom-electron attraction
in a covalent bond (pg 312)
the more positive EN, the stronger the attraction
(all values are positive)
Chemistry 1421
34
Brown & Lemay
Watkins
Chapter 8
Electronegativity - Fig. 8.6
EN shows the same trend as EA (stronger L→R, weaker T→B)
F = 4.0 highest
Electronegativity measures
H + Second Row: MEMORIZE
atom-electron attraction in
a covalent bond
2.1
H
Li Be B C N O F
1.0 1.5 2.0 2.5 3.0 3.5 4.0
Knowing these numbers will help
you understand many organic and
biological molecules and reactions.
Chemistry 1421
35
Brown & Lemay
Watkins
Chapter 8
Lewis Structures
A Lewis structure is a graphic
representation of a molecule or
molecule ion which
1. shows which atoms are connected
(bonded) together;
2. shows the approximate distribution of
valence shell electrons.
There are seven (7) steps to a correct
Lewis Diagram
Chemistry 1421
36
Brown & Lemay
Watkins
Chapter 8
Drawing Lewis Structures: Step 1
Counting Valence Shell Electrons (VSE)
VSE in an atom = Group Number (GN)
SumVSE for all atoms, count pairs
CO2 => 4 + 2(6) = 16 (8 pairs)
SO3 => 6 + 3(6) = 24 (12 pairs)
add electrons for anionic molecular ions
e.g., CO32- = 4 + 3(6) + 2 = 24 (12 pairs)
e.g., SO42- = 6 + 4(6) + 2 = 32 (16 pairs)
subtract electrons for cationic molecular ions
e.g., NH4+ = 5 + 4(1) - 1 = 8 (4 pairs)
e.g., ArH+ = 8 + 1 – 1 = 8 (4 pairs)
Chemistry 1421
37
Brown & Lemay
Watkins
Chapter 8
Drawing Lewis Structures: Step 1
Counting Valence Shell Electrons (VSE)
VSE in an atom = Group Number (GN)
All Valence Shell Electrons must be
shown in the Lewis Structure
Chemistry 1421
38
Brown & Lemay
Watkins
Drawing Lewis Structures: Step 2
Chapter 8
(connectivity)
Arrange Atomic Symbols for Bonding
– Bonded atoms must be contiguous on the paper;
– Definition: CA (Central atom)
an atom in the molecule which is covalently bonded to
two or more other atoms;
– Definition: PA (Peripheral atom)
an atom in the molecule which is covalently bonded to
only one other atom.
Chemistry 1421
39
Brown & Lemay
Watkins
Chapter 8
Drawing Lewis Structures: Step 2
(connectivity)
Arrange Atomic Symbols (e.g., CO32-, SO42-)
– Guidelines for arranging atoms for bond connections
• Least electronegative = Central Atom (CA)
• Most electronegative = Peripheral Atoms (PA)
• Monovalent atoms (F, H) = Always PA
– Sometimes: connectivity from formula (HCN)
– Positions of PAs around CA not significant
O C O
O
Chemistry 1421
O
O C O
O
O
C
40
O
O S O
O
O
O
O
S
O
O
Brown & Lemay
Watkins
Chapter 8
Drawing Lewis Structures: Step 3
(covalent bond formation)
Attach Atoms with single bonds
use one VSE pair per bond
S = CA
SO4
O = PA
32 e- (16 pairs)
2-
O
O
S
O
O
16 pairs - 4 pairs = 12 pairs left
Chemistry 1421
41
Brown & Lemay
Watkins
Chapter 8
Drawing Lewis Structures: Step 4
(lone pair distribution)
a. Complete Octets for peripheral atoms
especially C, N, O, F (but not H)
2-
SO4
12 pairs left
12 LP
0 pairs left
O
O
S
O
O
b. Give any remaining electron pairs to CA
(which must have empty V.S. orbitals)
Chemistry 1421
42
Brown & Lemay
Watkins
Chapter 8
Drawing Lewis Structures: Step 5
(Modify?)
Calculate Formal Charge on each atom
FC = GN - (# BP) - 2(# LP)
[-1]
O
O
[+2]
S
O[-1]
SO42-
6 - (4) - 2(0) = +2
6 - (1) - 2(3) = -1
O[-1]
(all 4 oxygen atoms)
Total Charge = Sum of All Formal Charges
[-1]
S:
O:
-2 = FC(S) + 4FC(O) = (+2) + 4(-1)
Chemistry 1421
43
Brown & Lemay
Watkins
Chapter 8
Drawing Lewis Structures: Step 5
(Formal Charge)
For each atom in the molecule:
• Maximum FC = Group Number
• Minimum FC = Group Number - 8
• FC must be consistent with electronegativity:
– F, O should never have FC > 0
– Least electronegative atom (usually a CA)
generally has FC 0
– Most electronegative atom (usually a PA)
generally has FC  0
Atoms in molecules have charges close to zero
(Pauling’s principle of Electroneutrality)
Chemistry 1421
44
Brown & Lemay
Watkins
Chapter 8
Drawing Lewis Structures: Step 6
(Modify Initial Lewis Structure)
[-1]
O
[+2]
O
S
[-1]
O[-1]
O[-1]
The initial Lewis structure can be
modified by electron pushing if
1. a central atom has less than an octet
of valence shell electrons;
2. formal charges are too high or are
inconsistent with electronegativities.
Electron pushing: move a lone pair on a peripheral
atom into bonding position, or vice-verse.
Chemistry 1421
45
Brown & Lemay
Watkins
Drawing Lewis Structures: Step 6
Chapter 8
(Modify Initial Lewis Structure)
FC = GN
- (# BP)
- 2(# LP)
[-1]
O
[-1]
[-1]
[+2]
O
S
[-1]
O
O
O
Bond Order 2
O
[0]
[0]
[0]
S
O
Bond Order 1
O
[-1]
[-1]
Each O Valence Shell retains octet
S Valence Shell now contains 12 electrons
could contain up to 18 electrons in all 9 orbitals
Bond Order: number of bond pairs shared between two
atoms
Chemistry 1421
46
Brown & Lemay
Watkins
Drawing Lewis Structures: Step 6
Chapter 8
(Modify Initial Lewis Structure)
FC = GN
- (# BP)
- 2(# LP)
[-1]
O
[-1]
[-1]
[+2]
O
S
[-1]
O
O
O
Bond Order 2
O
[0]
[0]
[0]
S
O
Bond Order 1
O
[-1]
[-1]
Does the sulfate ion really have two different kinds of
bonds? Does the sulfate ion really have two different
kinds of oxygen atoms? No! All 4 oxygen atoms and
all 4 S-O bonds are identical.
Chemistry 1421
47
Brown & Lemay
Watkins
Drawing Lewis Structures: Step 7
Chapter 8
(Resonance)
Resonance: two or more equivalent Lewis structures showing
moving double bonds and lone pairs (but not atoms). Each Lewis
structure is a “snapshot” of the electron positions.
O
O S O
O
O
O S O O S O
O
O
O
O
O
O
O S O O S O O S O
O
Chemistry 1421
O
O
48
AVERAGES
Formal Charge
S = (0+0+0+0+0+0)/6 = [0]
O = -3/6 = [-1/2]
Bond Order
S-O = 9/6 = 1.5
Brown & Lemay
Watkins
Drawing Lewis Structures: Step 7
Chapter 8
(Resonance)
Resonance: two or more equivalent Lewis structures showing
moving double bonds and lone pairs (but not atoms). Each Lewis
structure is a “snapshot” of the electron positions.
O
O S O
O
O
O S O O S O
O
O
O
O
O
O
O S O O S O O S O
O
Chemistry 1421
O
O
49
AVERAGES
Formal Charge
S = (0+0+0+0+0+0)/6 = [0]
O = -2/4 = [-1/2]
Bond Order
S-O = 6/4 = 1.5
Brown & Lemay
Watkins
Chapter 8
Summary
Lewis Diagrams
1. Count Valence Shell Electrons
2. Arrange atoms appropriately
3. Insert single bond pairs
4. Distribute lone pairs
5. Calculate formal charges
6. Modify if necessary
7. Resonance?
Homepage > Extra > Lewis Structures
Chemistry 1421
50
Brown & Lemay
Watkins
Chapter 8
Carbonate Ion
1. Count VSE:
4+3(6)+2=24 (12 pairs)
2. Connectivity (C=2.5, O=3.5)
3. Bond
12 – 3 = 9 LP
4. Lone Pairs on PAs
9-3(3)=0 pairs left
5. Formal Charges
C: 4-3-2(0)=+1
O: 6-1-2(3)=-1
Chemistry 1421
51
CO32[-1]
O
O
[-1]
C
[+1]
O
[-1]
Brown & Lemay
Watkins
Chapter 8
Carbonate Ion
CO32[-1]
6. Modify (push electrons)
to complete C octet
to reduce formal charges
7. Resonance
O
O
Chemistry 1421
C
O
[-1]
[+1]
[-1]
Averages
FC(C) = 0/3 = 0
FC(O) = -2/3 = -0.67
BO(C-O) = 4/3 = 1.33
O
C
O
[-1]
O
C
O
[0]
[0]
O
O
52
O
C
O
[-1]
O
O
O
C
O
Brown & Lemay
Watkins
Chapter 8
More Complex Molecules
Most molecules have more than one Central
Atom.
For many formulas, there is more than one
way to connect atoms (isomers)
Example: C2H6O
There are two completely different
isomers with this formula:
ethyl alcohol (b.p. 78 oC; l at 25 oC)
dimethyl ether (b.p. -24.8 oC; g at 25 oC)
Chemistry 1421
53
Brown & Lemay
Watkins
Chapter 8
More Complex Molecules
ethyl alcohol (b.p. 78oC)
C2H6O = CH3CH2OH
2(4) + 6(1) + 1(6) = 20 (10 pairs)
1. Count VSE
2. Connectivity
3 CAs (C, C, O), 6 PAs (H)
10 – 8 BP = 2 LP
3. Form Bonds
LP distribution:
4. Lone Pairs
H
Chemistry 1421
H
H
C
C
H
H
O
not on PAs (why?)
so must be on CA
but not C (why?)
so must be O
H
54
Brown & Lemay
Watkins
Chapter 8
More Complex Molecules
ethyl alcohol (b.p. 78oC)
C2H6O = CH3CH2OH
5. Formal Charges
6. Modify
7. Resonance
H
Chemistry 1421
H
H
C
C
H
H
O
H
No modifications
necessary:
No large FCs
Each H has duet
Each C has octet
The O has octet
No resonance:
No double bonds
both C: 4 – 4 – 2(0) = 0
all H: 1 – 1 – 2(0) = 0
O: 6 – 2 – 2(2) = 0
55
Brown & Lemay
Watkins
Chapter 8
More Complex Molecules
dimethyl ether (b.p. -24.8oC)
C2H6O = CH3OCH3
2(4) + 6(1) + 1(6) = 20 (10 pairs)
1. Count VSE
2. Connectivity
3 CAs, 6 PAs
10 – 8 BP = 2 LP
3. Form Bonds
LP distribution:
4. Lone Pairs
H
H
C
H
Chemistry 1421
not on PAs
so must be on CA
but not C
so must be O
H
O
C
H
H
56
Brown & Lemay
Watkins
Chapter 8
More Complex Molecules
dimethyl ether (b.p. -24.8oC)
No modifications
C2H6O = CH3OCH3
necessary:
No large FCs
Each H has duet
Each C has octet
The O has octet
No resonance:
No double bonds
5. Formal Charges
6. Modify
7. Resonance
H
H
C
H
Chemistry 1421
H
O
C
H
H
both C: 4 – 4 – 2(0) = 0
all H: 1 – 1 – 2(0) = 0
O: 6 – 2 – 2(2) = 0
57
Brown & Lemay
Watkins
Chapter 8
More Complex Molecules
5 C-H single bonds
1 C-C single bond
1 C-O single bond
1 O-H single bond
78oC)
ethyl alcohol (b.p.
C2H6O = CH3CH2OH
6 C-H single bonds
o
dimethyl ether (b.p. -24.8 C) 0 C-C single bond
2 C-O single bonds
C2H6O = CH3OCH3
0 O-H single bond
H
H
Chemistry 1421
C
H
H
O
C
H
H
H
58
H
H
C
C
H
H
O
H
Brown & Lemay
Watkins
Chapter 8
More Complex Molecules
acetate ion
C2H3O2- = CH3CO21. Count VSE 2(4) + 3(1) + 2(6) + 1 = 24/12 pairs
2. Connectivity 2 CAs, 5 PAs
3. Form Bonds 12 – 6 = 6 LP
4. Lone Pairs
Most electronegative PAs
H
H
O
C
C
O
H
Chemistry 1421
59
Brown & Lemay
Watkins
Chapter 8
More Complex Molecules
C1: 4 – 4 - 2(0) = 0
C2: 4 – 3 - 2(0) = +1
H: 1 – 1 - 2(0) = 0
O: 6 – 1 - 3(2) = -1
Modifications:
Reduce FC(C&O)
Complete octet(C)
acetate ion
C2H3O2- = CH3CO25. Formal Charges
6. Modify
[-1]
H
[-1]
H
O
O
[-1]
H
C
C
[0]
H
O
C
[+1]
O
[0]
H
H
Chemistry 1421
C
60
Brown & Lemay
Watkins
Chapter 8
More Complex Molecules
acetate ion
C2H3O2- = CH3CO2-
Averages:
FC(C2) = 0
FC(O) = -1/2
BO(C-O) = 1.5
7. Resonance
H
H
O
C
C
H
[-1]
O
H
H
C
H
O
C
C
O
[-1]
H
O -0.5
C
H
-0.5
O
H
Chemistry 1421
61
Brown & Lemay
Watkins
Chapter 8
More Complex Molecules
difluorodioxyxenon
F2XeO2
2(7) + 8 + 2(6) = 34 (17 pairs)
1. Count VSE
2. Connectivity 1 CA, 4 PAs
3. Form Bonds 17 - 4 BP = 13 LP
Complete octets of PAs
4. Lone Pairs
13 – 12 = 1 LP
Last LP on Xe (25 VS orbitals)
F
O
Xe
O
F
Chemistry 1421
62
Brown & Lemay
Watkins
Chapter 8
More Complex Molecules
difluorodioxyxenon
F2XeO2
5. Formal Charges
6. Modify
7. Resonance (none)
Xe: 8 – 4 – 2(1) = +2
O: 6 – 1 – 2(3) = -1
F: 7 – 1 – 2(3) = 0
[0]
[0]
F
F
[-1]
O
Chemistry 1421
[+2]
Xe
F[0]
[-1]
[0]
O
[0]
O
Xe
[0]
O
F[0]
63
Brown & Lemay
Watkins
Chapter 8
Summary
Lewis Diagrams
1. Count Valence Shell Electrons
2. Arrange atoms appropriately
3. Insert single bond pairs
4. Distribute lone pairs
5. Calculate formal charges
6. Modify if necessary
7. Resonance?
Homepage > Extra > Lewis Structures
Chemistry 1421
64
Brown & Lemay
Watkins
Chapter 8
Covalent Bond Energy
C
C
Chemistry 1421
Bond Order
Bond Strength (the energy required to break a
bond) depends on:
The specific atoms forming the bond:
C H 413 kJ/mol
C C 348 kJ/mol
The bond order:
C C 348 kJ/mol
C 614 kJ/mol
C 839 kJ/mol
Bond Strength
65
Brown & Lemay
Watkins
Chapter 8
Covalent Bond Energy
Bond Strength or "Dissociation Energy" D is the
energy absorbed (as heat, light, etc.) to break a
covalent bond.
D = qp or hn
Bond breaking is always endothermic
Chemistry 1421
66
Brown & Lemay
Watkins
Chapter 8
Covalent Bond Energy
Bond Energy is the energy emitted (heat, light, etc.)
to make a covalent bond.
qp or hn = –D
Bond making is always exothermic
Chemistry 1421
67
Brown & Lemay
Watkins
Chapter 8
Covalent Bond Energy
Bond breaking is always endothermic
Bond making is always exothermic
Chemistry 1421
68
Brown & Lemay
Watkins
Chapter 8
Approximate Reaction Enthalpy
The energy of a reaction is the sum of all
bond breakings and bond makings.
2H2(g) + O2(g) → 2H2O(g) DHrx = -483.64 kJ
D(H-H)
2[-D(O-H)]
2[-D(O-H)]
D(O=O)
D(H-H)
Chemistry 1421
2(+436) + (+495) + 4(-463) = -485 kJ
69
Brown & Lemay
Watkins
Chapter 8
Approximate Reaction Enthalpy
1. Write and balance the chemical reaction;
2. Draw the Lewis structure of each reactant and
each product;
3. List all reactant and product bond orders;
4. Look up the bond strength for each bond order;
5. Add all reactant bond strengths;
6. Subtract all product bond strengths.
Chemistry 1421
70
Brown & Lemay
Watkins
Chapter 8
Electron Counting
There are several ways to partition
the electrons in a molecule
1. Molecular Valence Shell Electrons
(LP + BP)
2. Atomic Valence Shell Electrons
(noble gas configuration?)
3. Atomic Formal Charges
4. Atomic Partial Charges
5. Atomic Oxidation Numbers
Chemistry 1421
71
Brown & Lemay
Watkins
Chapter 8
Electron Counting
Molecular Valence Shell Electrons
MVSE = S(GN) - (Total Charge)
Example: SO42–
MVSE = (6) + 4(6) – (–2) = 32
Example: CH3NH3+
MVSE = 4 + 6(1) + 5 – (+1) = 14
Example: C2H6O
MVSE = 2(4) + 6(1) + 6 – (0) = 20
Chemistry 1421
72
Brown & Lemay
Watkins
Chapter 8
Electron Counting
Atomic Valence Shell Electrons
(each atom in the Lewis structure)
AVSE = 2(BP + LP)
Example: SO42– (any of 10 resonance form)
AVSE(O) = 2(2+2) = 2(1+3) = 8
AVSE(S) = 2(5+0) = 10 or 2(6+0) = 12
×6
×4
Chemistry 1421
73
Brown & Lemay
Watkins
Chapter 8
Electron Counting
Atomic Formal Charge
(each atom in the Lewis structure)
AFC = GN - (BP) - 2(LP)
Example : SO42– (average over 10 resonance forms)
AFC(S) = +0.40
AFC(O) = –0.60
×6
×4
Chemistry 1421
74
Brown & Lemay
Watkins
Chapter 8
Electron Counting
Atomic Partial Charge
(each atom in the Lewis structure)
APC = AFC + d
The formal charge is altered by taking into account the
differential electronegativity of the A-B pair connected
by a covalent bond of order “BO”:
cA
dA = BO
Chemistry 1421
A
BO
½(cA+cB) – cA
B
dB = BO
½(cA+cB)
75
cB
½(cA+cB) – cB
½(cA+cB)
Brown & Lemay
Watkins
Chapter 8
Electron Counting
Atomic Partial Charge
(each atom in the Lewis structure)
APC = AFC + d
Example: SO42-: cS = 2.5, cO = 3.5
(average over 10 resonance forms):
<BO> = 1.40, <FC(S)> = +0.40, <FC(O)> = -0.60
for each S-O bond, dS = +0.233, dO = -0.233
APC(S) = +1.33, APC(O) = –0.83
From Gaussian 09, RHF/6-311g++:
APC(S) = +1.74, APC(O) = –0.94
Chemistry 1421
76
Brown & Lemay
Watkins
Chapter 8
Electron Counting
Atomic Oxidation Number
(each atom in the Lewis structure)
AON = GN - # assigned electrons
Assign...
1. ... both electrons of a lone pair;
2. ... both electrons of a covalent bond to the
most electronegative atom of the bonded pair;
3. ... one electron of a covalent bond to each
atom if they are identical.
Chemistry 1421
77
Brown & Lemay